Protection switching is a general term used to describe a technique for maintaining communication services in the event of a failure in certain equipment, for example, within a given telecommunications network. Protection switching is commonly used in electronic, optical or electronic/optical telecommunications networks. A failure may occur in either the electronic or optical equipment or in both. The failure also may occur in the physical medium used to transmit the optical signal, such as through a break in a fiber optic cable. This break may be caused by external aggressions such as through the misuse of a backhoe, a ship's anchor, a shark bite or an operator pulling out the wrong connection or turning off the wrong switch.
Referring to FIGS. 1–4, existing telecommunications systems use various protection switching techniques. The telecommunications system typically includes a physical medium or line 10 with electronic and optical equipment 12 located at regular intervals along the medium 10 and terminal equipment 14, 15 at each end of the medium 10. Transmitting terminal equipment 14 is used to prepare signals for transmitting information over the medium 10, and receiving terminal equipment 15 is used to recover the information at the receiving end of the medium. The electronic and optical equipment 12 is used in connection with the physical medium 10 to enable transmission, for example, by amplifying or regenerating the signals. Although only one direction of transmission is shown with transmitting terminal equipment at one end and receiving terminal equipment at the other end, the telecommunications system includes both receiving and transmitting terminal equipment at each end to provide transmission in both directions.
As shown in FIG. 1, many telecommunications systems protect the main physical medium 10 by providing a separate physical medium 10a installed on a physically separate route, where simultaneous damage to both media is very unlikely. Various protection-switching architectures exist to transfer communications traffic from one medium to the next in the event of a failure in the former. Spontaneous failures may also occur in the complex electronic and optical equipment 12. Where electrical and optical equipment 12 is located at regular intervals, such equipment is frequently protected in conjunction with the medium 10 by moving the traffic to the separate medium 10a, which is equipped with its own separate electronic and optical equipment 12a. In the existing system, the transmitting terminal equipment 14 can be coupled to the lines or physical media 10, 10a with a splitter 11 to provide duplicate signals over the lines 10, 10a. The receiving terminal equipment 15 can be coupled to the lines or physical media 10, 10a with a selector 13 (e.g., a 2:1 switch) to select the signal from either of the lines 10, 10a. This protection switching technique is often referred to as 1+1 line protection.
Failures also may occur in the terminal equipment 14, 15 at the ends of the medium 10. Several methods currently exist to protect terminal equipment against failure. One known method provides separate terminal equipment 14a, 15a, as shown in FIG. 2, for the separate, physically diverse transmission medium 10a. In this existing system, the splitter 11 is outside the transmitting terminal equipment 14, 14a to provide the same data stream to both the transmitting terminal equipment 14 and the spare transmitting terminal equipment 14a. The selector 13 is located outside of the receiving terminal equipment 15, 15a to select the data stream from either the receiving terminal equipment 15 or the spare receiving terminal equipment 15a. This protection switching technique is sometimes called 1+1 combined terminal and line protection.
This 1+1 combined terminal and line protection method requires full duplication of the terminal equipment. Because the terminal equipment can represent a large fraction of the total cost of a transmission system, this duplication of equipment adds considerable cost to the overall system. Another disadvantage to this method is the amount of physical space needed to house the extra sets of terminal equipment. Effectively, each terminal facility is likely to be double in size.
In one type of telecommunications system, shown in FIG. 3, separate wavelength processing devices 16, 18 are provided for separate information channels provided over the same physical medium 10, for example, through wavelength division multiplexing (WDM) of multiple channels over a single optical fiber. In a WDM system, a wavelength combining device 20 (e.g., a multiplexer) combines the separate wavelengths (λ1, λ2, . . . λN) from the respective wavelength processing device transmitters 16 and a wavelength separating device 22 (e.g., demultiplexer) separates the wavelengths (λ1, λ2, . . . λN) to be received by the respective wavelength processing device receivers 18. Where protection against a failure of the transmission medium is desired in such cases, a separate cable 10a with a similar number of constituent fibers is provided. Each fiber in one cable can carry multiple wavelengths, acting to protect its counterpart in the other cable. To provide 1+1 combined terminal and line protection in this case, full duplication of the terminal equipment requires duplicative wavelength processing devices 16a, 18a for each and every wavelength, again representing a considerable expense and increased floor space in the terminal facility.
Where a system includes a plurality of channels and associated wavelength processing devices 16, another approach in protecting terminal equipment is to provide a single spare wavelength processing device 16a, as shown in FIG. 4. Through switching circuits (e.g., splitters 11, selector 24, splitter 25 and selectors 26), the spare wavelength processing device 16a can be substituted for any one of the wavelength processing devices 16 at one end of the transmission path. In this system, the wavelength combining device 20 combines the N wavelengths from the wavelength processing devices 16 and the spare wavelength processing device 16a when it is substituted. This protection switching technique is commonly known as 1×N or 1:N equipment protection.
According to a variation of this method, as shown in FIG. 5, the physical transmission medium 10 includes an additional transmission path, such as an additional fiber in a cable or an additional wavelength on the same fiber in a WDM system. The additional fiber (or wavelength) is used to interconnect the spare wavelength processing devices 16a, 18a at the two ends of the system. A N+1 wavelength combining device 28 combines the wavelengths from the wavelength processing devices 16 in addition to the additional wavelength from the spare wavelength processing device 16b. The spare wavelength processing devices 16a, 18a at each end are used to protect against a failure of any one of the wavelength processing devices 16 and 18, using switching arrangements at the two ends of the transmission system. This protection switching technique is sometimes referred to as the 1×N end-to-end path protection. However, this method may not protect against damage to the physical transmission medium 10, such as a cable, unless N separate diverse routes are provided, which is practically uneconomical.
In many cases, the terminal equipment at the ends of the system is used to identify failures in the transmission medium and to initiate a protection switch. This is especially true as the length of the transmission path increases. In some systems, the presence or absence of a signal is one way of detecting a failure on the medium without the use of terminal equipment. In very long systems, however, the presence or absence of a signal does not indicate the “health” of the transmitted signal and is not necessarily an indication that the medium is capable of transmitting the signal with sufficient integrity.
Use of terminal equipment to monitor performance allows actual monitoring of bit error rate (BER) performance in a digital system. The BER verifies the measure of performance typically of importance to customers. Where the transmission method is WDM, detection of media failures with the terminal or wavelength processing device associated with each individual wavelength is desired. In general, it is desirable to monitor both diversely routed cables—to identify failures in the channel carrying traffic and to assure that the redundant channel is ready to carry service in the event the other cable fails. To provide this type of monitoring in existing systems requires separate terminal or wavelength processing devices on every wavelength of each of the two (or more) diversely routed cables. This again leads to inordinate costs and floor space.
In view of the disadvantages of the protection switching techniques mentioned above, there is a need for a protection switching system and method in telecommunications systems, especially WDM systems, which reduces the number of spare wavelength processing devices and assures proper operation of all physical media using the capabilities of the terminal equipment.